Many hydraulic units of the variable displacement type have a rotating cylinder block with pistons axially movable therein. The displacement of the hydraulic unit is proportional to the stroke of the pistons within the cylinder block. Where the hydraulic unit is of the axial piston type, the pistons or piston slippers engage a tiltable swashplate to vary the stroke of the pistons. When the swashplate is perpendicular to the axis of the cylinder block, the swashplate is in the neutral or a zero displacement position and the hydraulic unit has no output.
In order to maintain the swashplate in its zero displacement position when no control forces are applied thereto, various swashplate centering mechanisms have been utilized. Generally such centering mechanisms are a plurality of springs which apply opposite biasing forces on the swashplate at points spaced from the tilt axis of the swashplate. Hann et al U.S. Pat. No. 3,359,727 issued Dec. 26, 1967 shows the centering springs to be placed within hydraulic servo mechanisms which are utilized to control the tilt of the swashplate. Such springs may be of a short unstressed length or have a length limiting means to prevent engagement of the spring with the servo piston until the swashplate tilts toward the servo cylinder containing the spring. This, however, requires, very accurate spring lengths or adjustment thereof to minimize backlash and insure that the centering force of a given spring does not start until the swashplate is tilted toward that spring but still assures that the spring starts to act on the swashplate exactly when the swashplate is in the zero displacement position.
Another version of a swashplate leveling and holddown device is taught in Forster et al U.S. Pat. No. 4,142,452 issued Mar. 6, 1979 teaching a cradle type swashplate resting in a roller bearing pocket and having four swashplate positioning devices located in the corners of the hydraulic unit housing. In one embodiment of Forster, all four mechanisms are servo pistons with prestressed springs such as mentioned above. In another embodiment of Forster, two of the locating mechanisms, located on one side of the tilt axis of the swashplate, are servo units while the two locating mechanisms located on the opposite side of the tilt axis are spring units. Since the spring units are only on one side of the tilt axis, the spring units cannot be used as a leveling device but can only counterbalance the axial biasing force of the servo cylinders on the opposite side of the tilt axis. Even in the first embodiment where the four spring servos apply an axial holddown force on the cradle swashplate, that is to hold the cradle swashplate against its roller bearings, the four springs must be critically dimensioned and adjusted during assembly to provide a spring centering function on the swashplate.
In the prior art structures, such as Forster, in order to have counter-balancing spring centering, it is quite critical that the springs have the same axial force characteristics which requires adjustment and the associated extra parts and assembly steps. Such adjustment must compensate for leveling and backlash. Without complete backlash adjustment, accurate leveling cannot be achieved. Furthermore, use causes a spring to lose its spring rate or take a set and this characteristic alters any previous adjustment. Even though the spring rate loss characteristic may only be a few percent of the total force supplied by the spring, any difference in spring rate loss has a major effect upon the centering forces of the spring and thus prevents swashplate from centering at its zero displacement position.